In recent years, with the rapid spread of information-related devices such as personal computers, video cameras and mobile phones, the importance in developing improved batteries for use as power supplies in such devices has been more recognized. In the automotive industry as well, advances are being made in the development of high-power and high-capacity batteries for electric cars and hybrid cars. Of the various types of batteries that exist, particular attention is being paid to lithium batteries on account of their high energy density and power.
In a lithium battery, the positive electrode active material is generally a lithium metal complex oxide having a layered structure of, for example, lithium nickelate and lithium cobaltate. The negative electrode active material is typically, for example, a carbon material capable of intercalating and deintercalating lithium ions, lithium metal, or a lithium alloy. The electrolyte interposed between the positive electrode and the negative electrode is generally, for example, an electrolyte solution containing in which lithium salt has been dissolved, or a lithium-containing solid electrolyte.
Although lithium batteries do have, as noted above, an excellent energy density and power, the rising price of lithium associated with increased demand for lithium batteries and the fact that lithium reserves are limited serve as bottlenecks to mass production and the scaling up of production.
Hence, research is being carried out on sodium batteries, which use sodium—natural deposits of which are abundant and which is low in cost—instead of lithium (see, for example, Patent Document 1 and Non-Patent Documents 1 to 4).
For example, Patent Document 1 discloses positive electrode active materials for nonaqueous electrolyte secondary batteries of the formula MaxMbyP2O7 (where Ma is Na, Li, Ca or Mg, Mb is a transition metal that is stably present at a valence of 4 and above, 0≦x≦4, 0.5≦y≦3, and 6≦z≦14). What was in fact produced and evaluated in the working examples of Patent Document 1 was MoP2O7.
Patent Document 1: Japanese Patent Application Publication No. 2004-158348
Patent Document 2: Japanese Patent Application Publication No. 2005-183395
Non-Patent Document 1: Abstract #389, 218th ECS Meeting (2010), The Electrochemical Society
Non-Patent Document 2: LiBD-5 2011—Electrode materials—Arcachon, France; 12-17 Jun. 2011
Non-Patent Document 3: Electrochemistry Communications, 12 (2010), 355-358
Non-Patent Document 4: Nature Materials DOI; 10.1038/NMAT2920
Non-Patent Document 5: Richiumu niji-denchi (Lithium secondary batteries), written and edited by Zenpachi OGUMI (Ohmsha), p. 77